1. Field of the Invention
The present invention relates to an optical compensatory sheet producing method and apparatus, thermal treating method and apparatus, and dust removing method and apparatus. More particularly, the present invention relates to an optical compensatory sheet producing method and apparatus in which an optical compensatory sheet can be obtained with high quality for good contrast and color in a liquid crystal display unit, and thermal treating method and apparatus, and dust removing method and apparatus for the optical compensatory sheet.
2. Description Related to the Prior Art
A liquid crystal display unit (LCD) is characterized in having a shape with a small thickness, being lightweight and using low electric power in comparison with a cathode ray tube (CRT) to display an image. LCD is used widely as a component incorporated in a portable type of word processor and a desk-top type of personal computer. In LCD, twisted nematic liquid crystal is used. In general, LCD include a liquid crystal cell and a pair of polarizing plates between which the liquid crystal cell is disposed. The liquid crystal cell is constituted by a liquid crystal panel and an electrode for applying voltage to the between the orientation layers. To display an image in LCD, there are two modes include a birefringence mode and an optically rotary mode.
A super twisted nematic LCD (hereinafter referred to as STN-LCD) utilizing the birefringence mode uses a super twisted nematic liquid crystal showing a twisted angle more than 90 degrees and having steep electro-optical characteristics. Such STN-LCD, therefore, has an advantage of giving display data of a large size by driving in time-sharing mode. However, the STN-LCD has disadvantages in providing good contrast only when set in the yellow mode (yellowish green/deep blue) or blue mode (blue/pale yellow). A phase difference plate (uniaxial stretched polymer film or compensatory liquid crystal cell) is required to give black-and-white display.
TN-LCD, when set in an optically rotary mode, shows quick response (as quick as several tens of milliseconds) and high display contrast. Hence, the optical rotary mode has a number of advantages compared with the birefringence mode or other modes. However, TN-LCD has disadvantages that color or contrast on display varies with changes in viewing angle to LCD, and its display characteristics are not comparable to display characteristics of CRT.
In order to improve the viewing angle characteristics (i.e., to enlarge the viewable angle), arrangement of a phase difference film (optical compensatory sheet) between a pair of polarizing plates and the liquid crystal cell has been known. The optical compensatory sheets show no optical effect when an LCD is seen in the direction vertical to the screen of the display because phase difference in the direction perpendicular to the surface of LCD is almost zero. However, the optical compensatory sheet serves to compensate for phase difference (depending upon wavelengths of light) that occurs when LCD is viewed in an oblique direction.
JP-A 6-075115, U.S. Pat. No. 5,506,706 (corresponding to JP-A 6-075116), and JP-A 4-276076 disclose an optical compensatory sheet having the negative birefringence and inclined optical axis. In more detail, the disclosed sheet is produced by stretching polymer such as polycarbonate or polyester, and has the directions of the main refractive indices which are inclined from the normal of the sheet. To prepare the above sheet by the stretching treatment, extremely complicated treatments are required. Therefore, an optical compensatory sheet of a large surface are cannot be easily produced according to the disclosed process.
Also, an optical compensatory sheet comprising liquid crystalline polymer is known. For instance, U.S. Pat. No. 5,064,697 (corresponding to JP-A 3-009326) and JP-A 3-291601 disclose an optical compensatory sheet for LCD which is prepared by applying a coating of polymer showing liquid crystal property on an orientation layer provided on a support film. However, polymers having a liquid crystalline property have a shortcoming in low productivity and are unsuitable for mass production, because it needs orientation by ripening for a long time at high temperature. JP-A 5-215921 discloses the use of a birefringence plate (optical compensatory sheet) comprising a support and a polymerizable rod-like compound showing liquid crystal property and positive birefringence. The birefringence plate is prepared by applying a coating of solution of the rod-like compound to the support and curing the compound with heat. The cured layer of the prior document, however, does not show negative birefringence. Hence, the resulting compensatory sheet cannot enlarge satisfactorily the viewing angle in all directions.
U.S. Pat. No. 5,646,703 (corresponding to EP-A 0 646 829) discloses an optical compensatory sheet greatly enlarging the viewing angle in all directions. The optical compensatory sheet has a representative structure comprising a transparent support, an orientation layer thereon, and a layer of discotic liquid crystalline compound provided on the orientation layer.
To produce an optical compensatory sheet with a large area, a transparent resin film with a large area must be coated with a discotic liquid crystal compound in a state substantially free from defects. So it has been conceived to reduce the number of the defects in consideration of enlarging LCD.
To produce the optical compensatory sheet, a rubbing process rubs a surface of a resin layer overlaid on the resin film for forming an orientation layer by use of a rubbing sheet (of fabric). This creates dust such as minute particles of resin. The dust becomes collectively stuck on a surface of the orientation layer, to create the defects (uneven orientation or local failure in orientation) in an oriented state of a liquid crystal layer of the optical compensatory sheet. When LCD is provided with the optical compensatory sheet, the defects occur in a display state. To avoid failure in removal of dust, the rubbing sheet about a rubbing roller should be exchanged and renewed frequently. However, there is a problem in that occurrence of the defects gradually becomes frequent according to dust. Furthermore, renewal of the rubbing sheet must be periodical, and inconsistent to continuous operation of the manufacture.
JP-A 9-073081 discloses a producing method of an optical compensatory sheet. Transparent resin film in a continuous shape is coated with solution including resin for forming an orientation layer, then is subjected to a rubbing process. So the orientation layer is formed, then is coated with solution including discotic liquid crystalline compound. Then the coating is dried. The resin film is thermally treated next. The resin film is heated to discotic nematic forming temperature, to form a liquid crystal layer oriented at a predetermined angle of an orientation axis.
In FIG. 18, an example of thermal treating apparatus is depicted. There are plural air blow openings 102 disposed above and below resin film 101 as web, and arranged in an alternate manner with one another. The air blow openings 102 supply hot air, blow both surface of the resin film 101, and apply heat thereto. Exhaust openings 103 are opposed to respectively the air blow openings 102, and ejects the hot air from the resin film 101.
In the optical compensatory sheet, the liquid crystal layer must be oriented according to an orientation axis previously intended.
In the thermal treating apparatus according to the prior art, it is likely that offsetting of an orientation axis occurs in a width direction of the discotic liquid crystal layer. If the offsetting of the orientation axis is over a tolerable range, portions of the resin film must be discarded. There are problems in that yield of products of the optical compensatory sheet decreases to increase the manufacturing cost, and that efficiency in manufacturing operation becomes lower according to inspection of the offsetting of the orientation axis and the discarding operation.
In manufacturing the optical compensatory sheet, dust must be removed from the resin film by a dust removing apparatus. U.S. Pat. No. 4,577,362 (corresponding to JP-A 59-150571) discloses an example of dust removal in a dry type. A non-woven fabric or blade is pressed against a surface of the resin film. Also, JP-A 10-309553 discloses dust removal in which compressed air with high cleanness is supplied to blow the resin film and separate dust from the same. An exhaust opening sucks the dust by ejecting the air. Also, there is an apparatus characterized in that an ultrasonic vibrator is used to vibrate an air flow, which separates dust with higher shearing force for the purpose of efficient cleaning.
Also, JP-A 10-290964 discloses a dry type of dust removal in which electrostatic charge is used. Positive and negative ions of air are injected to neutralize the charge. Dust is separated and then eliminated by use of another flow or air.
Also, a wet type of dust removal is also known. There is a method in which the resin film is traveled through a cleaning liquid in a cleaning liquid bath, and an ultrasonic vibrator is driven to separate dust from the resin film in the cleaning liquid bath. Furthermore, U.S. Pat. No. 3,956,790 (corresponding to JP-B 49-013020) discloses a method in which the resin film is supplied with cleaning liquid, and then compressed air blows the resin film to such the dust.
Those cleaning methods are effective to dust of particles of at least 10 μm or tens of μm, or dust stuck weakly. However, it is experimentally found that the known cleaning methods are nearly ineffective to dust of particles of 10 μm or less, or dust stuck with strong adhesion.
To solve such problems. JP-B 5-050419 (corresponding to JP-A 62-060749) proposes a method in which a cleaning rod is pressed against the resin film to remove dust. For the cleaning rod, the resin film immediately after being coated with solvent is supplied. The cleaning rod is caused to rotate in reverse to the traveling to clear the dust while the solvent remains. According to the document, a space is formed between the resin film and the cleaning rod with a small thickness. Dust having a size over the space is stopped from passing the rod. Also, more minute dust is separated by transmission of shearing force in the solvent liquid, so that dust of a small size or strongly stuck dust can be removed effectively.
Also, JP-A 62-065872 discloses a dust removing method in which a blade with a sharp edge is used to contact the resin film.
However, problems arise in the method of JP-B 5-050419 (corresponding to JP-A 62-060749) and JP-A 62-065872. The cleaning rod or the like directly contact the surface of the resin film. The cleaning rod or the like is formed from super hard alloy resistant to abrasion, and has a very smooth surface. Damages or scratches may occur in the resin film if hard foreign matter or dust becomes squeezed between the cleaning rod and the resin film.